The present invention relates to an improvement in a dialing apparatus provided for an extension telephone set of a key telephone system.
In the case of a key telephone system, a key service unit provided with exchange and control functions is connected to at least one office line, and further a plurality of extension telephone sets are connected to the key service unit via a speech signal line and a control signal line, respectively. In general, the key service unit receives power from a commercial power supply, and each extension telephone set receives the commercial power from the key service unit via the signal line. Accordingly, when the commercial power fails in the key service unit, various functions of the key telephone systems are disabled.
In the key telephone system, however, since there exists a need for maintaining at least the functions of an individual telephone set even during power failure (i.e. the individual telephone set can be connected directly to the office line for providing telephone service), in some cases power failure extension telephone sets provided with the functions of not only an extension telephone set but also an individual telephone set are used.
FIG. 1 shows the configuration of a prior-art power failure extension telephone set. In the drawing a power failure extension telephone set 5 is connected to a key service unit 2 via a control signal line 3 and a speech signal line 4, and the key service unit 2 is connected to an office line 1. The key service unit 2 is provided with a line switching circuit 6. This line switching circuit 6 connects the output of a power supply circuit 29 to the signal lines 3 and 4 to supply power from the key service unit 2 to the telephone set 5 when commercial power is supplied, but connects the office line 1 directly to the signal lines 3 and 4, as shown in FIG. 1, when commercial power fails.
The power failure extension telephone set 5 is generally provided with a speech circuit 10 and a control circuit 19 (both incorporated in an ordinary extension telephone set and activated by power supplied through the key service unit 2) and a telephone circuit 15 and a dial signal transmit circuit 20 (both incorporated in an ordinary individual telephone set and activated by power supplied through the office line 1). In addition, this prior-art power failure telephone set 5 is provided with two switching circuits 13 and 17 for switching the function of the telephone set 5 from the extension telephone set to the individual telephone set or vice versa according to the power supply or power failure condition.
In the telephone set 5, when the commercial power is being supplied, a voltage stabilizer 12 is connected to the signal lines 3 and 4 via the switching circuit 13 to supply power from the key service unit 2 to the speech circuit 10 and the control circuit 19. Further, a handset 16 is connected to the speech circuit 10 via the switching circuit 17. Therefore, the power failure telephone set 5 functions as an extension telephone set.
On the other hand, when the commercial power fails, the switching circuits 13 and 17 are switched as shown in FIG. 1, so that the telephone circuit 15 and the dial transmit circuit 20 are connected to the signal lines 3 and 4 and further the signal lines 3 and 4 are directly connected to the office line 1 through the key service unit 2. In addition, the handset 16 is connected to the telephone circuit 15. Therefore, the power failure telephone set 5 functions as an individual telephone set.
In the prior-art power failure extension telephone set as described above, a push-button dial switch 18 of double contact-layer structure is adopted to transmit each different dial signal when the telephone set functions as an extension telephone set and as an individual telephone set, respectively. In more detail, the dial switch 18 is formed of two 3.times.4 switch matrix of double contact-layer structure, so that each contact layer functions for each of the extension telephone set and the individual telephone set. In other words, a first contact matrix 18a of the contact group for the extension telephone set is connected to the control circuit 19 for the extension telephone set, and a second contact matrix 18b of the contact group for the individual telephone set is connected to the dial signal transmit circuit 20 for the individual telephone set, separately.
In the prior-art power failure telephone set, since a dial switch 18 of complicated double contact-layer structure is adopted, there exists a problem in that erroneous dialing operation occurs due to malconnection when the dial switch 18 is depressed roughly as when corners of each dial button are depressed. To overcome this problem, it may be possible to adopt a highly reliable dial switch which will not cause malconnection even if the dial button is depressed at a corner thereof. In this case, however, another problem arises in that high quality manufacturing technology will be required and therefore the cost of the dial switch inevitably increases.